Assessing MR-linac radiotherapy robustness for anatomical changes in head and neck cancer

Abstract

The MR-Linac will provide excellent soft tissue contrast for on-treatment imaging. It is well known that the electron return effect (ERE) results in areas of increased and decreased dose at air/tissue boundaries, which can be compensated for in plan optimisation. However, anatomical changes may affect the quality of this compensation. In this paper we aim to quantify the interaction of anatomical changes with ERE in head and neck (H&N) cancer patients. Twenty patients treated with either 66 Gy or 60 Gy in 30 fractions were selected. Ten had significant weight-loss during treatment requiring repeat CT (rCT) and ten had PTVs close to the sinus cavity. Plans were optimised using Monaco to meet the departmental dose constraints and copied to the rCT and re-calculated. For the sinus patients, we optimised plans with full and empty sinus at both 0 T and 1.5 T. The effect of the opposite filling state was next evaluated. No clinically relevant difference between the doses in the PTV and OARs were observed related to weight-loss in 0 T or 1.5 T fields. Variable sinus filling caused greater dosimetric differences near the walls of the sinus for plans optimised with a full cavity in 1.5 T, indicating that optimising with an empty sinus makes the plan more robust to changes in filling. These findings indicate that current off-line strategies for adaptive planning for H&N patients are also valid on an MR-linac, if care is taken with sinus filling.

Assessing localized dosimetric effects due to unplanned gas cavities during pelvic MR‐guided radiotherapy using Monte Carlo simulations

“…The effect of the magnetic field during MRgRT on the dose deposition has been shown to be influenced by the magnetic field strength, the incident beam energy, the beam entrance angle, and the boundary shape . Studies show that clinically acceptable plans can be achieved with multiple beam configurations over the target, and it has even been suggested that dosimetric effects due to the magnetic field could be used to develop partial breast MRgRT treatments …”

“…During this time, gas or air is likely to appear in or pass through OARs, which would create deviations from the adapted dose distributions. In head and neck cancer patients, dosimetric effects due to ERE when the sinus cavity fills or clears have been shown to be of potential clinical concern . As larger changes are expected in other regions of the body, such as the digestive track, work should be done to understand the dosimetric effects of unplanned gas appearing in OARs during MRgRT.…”

“…5,6 The effect of the magnetic field during MRgRT on the dose deposition has been shown to be influenced by the magnetic field strength, the incident beam energy, the beam entrance angle, and the boundary shape. [7][8][9][10] Studies show that clinically acceptable plans can be achieved with multiple beam configurations over the target, [10][11][12] and it has even been suggested that dosimetric effects due to the magnetic field could be used to develop partial breast MRgRT treatments. 13 However, as typical MRgRT workflows include daily imaging and a certain degree of adaptation of the original plan, the patient can be lying on the treatment coach for up to 30 min before treatment delivery.…”

Emerging radiotherapy technologies and trends in nasopharyngeal cancer

“…Adaptive planning for NPC patients has been shown to significantly reduce the dose to the ipsilateral parotid gland [64]. Chuter et al [65] showed that it was feasible for an MR-linear accelerator to utilize current off-line strategies for adaptive planning for head and neck cancers.…”

Adaptive Radiotherapy: Moving Into the Future

“…Today, advances in imaging technology in the treatment room enable the ability to visualize, at each treatment fraction, anatomical, and in some cases functional, the response of the tumor and normal tissue to treatment. [23][24][25][26][27][28][29][30][31] This has dramatically changed the scope of adaptive radiotherapy. In addition to the increasing use of MR imaging, advances in image reconstruction for CBCT imaging, enabling 4D CBCT, breath-hold CT, and artifact reduction, has advanced the use of CBCT for adaptive radiotherapy, enabling dose calculations, detection of anatomical response, and improved understanding of physiological motion.…”